As the demand for wireless communications increases, and as users increasingly travel between different wireless communication systems, there is a corresponding increase in demand for roaming between wireless communication systems. Typically, a user subscribes to wireless communication services in a home region. A wireless communication service operator may program the wireless device with the subscriber's home system as a more-preferred wireless communication system. When the user of a wireless device travels from one geographic region to another, the wireless communication service operator may program the wireless device to select wireless services from among several wireless communication systems. Outside of the user's home system, the user may communicate by means of a roaming service with any of a number of available wireless communication services. A “roamer” is a wireless device requiring service in a system operated by a wireless communication service operator other than the home system.
The wireless device maintains a list of system identification (SID) elements stored in memory. This list of stored SID elements is referred to herein as the preferred roaming list (PRL). Some of the stored SID elements are “more-preferred” stored SID elements. More-preferred stored SID elements are associated with wireless communication systems that the home operator prefers such that the wireless device acquires the more preferred systems. Additionally, some of the stored SID elements in the PRL include “less-preferred”stored SID elements, such that the wireless device only acquires less-preferred stored SID elements if the more-preferred stored SID elements are not available according to a roaming algorithm as is known in the art. The PRL will likely contain numerous stored SID elements as well as corresponding acquisition information, which differ with respect to their SIDs and their geographic regions, but may include, for example, channel number, frequency block designator, and mode as is known in the art.
The PRL is maintained in such a manner that the wireless device can readily determine which stored SID elements are more-preferred or less-preferred within a common geographic region. The references to common geographic regions refer to regions where different stored SID elements share a common or overlapping coverage area. Additionally, the stored SID elements that provide service in a common geographic region are prioritized such that each stored SID element is ranked from a more-preferred stored SID element to a less-preferred stored SID element within the geographic region.
The wireless device attempts to acquire service from a desired wireless communication system through an acquisition process. According to the acquisition process, the wireless device selects a stored SID element from the PRL and attempts to receive corresponding SID information that is broadcast by the desired wireless communication system and received by the wireless device. Each base station broadcasts SID information over a control channel in order to uniquely identify the wireless system it is associated with. The wireless device determines whether it acquired the desired wireless communication system by comparing the received SID information with the stored SID element corresponding to the desired wireless communication system.
The wireless device determines whether it is roaming by comparing the stored SID elements corresponding to the home wireless communication system or systems programmed in the wireless device, with the SID information of the wireless communication system providing service that is broadcast by that system and received by the wireless device. The wireless device also uses the received SID information to determine the geographic region in which the wireless device is located by comparing the received SID information with the geographic regions associated with the corresponding SID elements in the PRL.
Once the wireless device identifies the current geographic area, the wireless device attempts to acquire service on a more-preferred stored SID element within the wireless device's current geographic region. If the acquired SID information belongs to a more-preferred stored SID element in the PRL, the wireless device camps on the more-preferred stored SID element as is known in the art. However, if the home system has a coverage hole or an area of interference causing a temporary loss of service, the wireless device may then attempt to roam outside of the user's home system. As a result of entering the coverage hole or the interference region, the wireless device cannot acquire the more-preferred stored SID element, i.e., the home wireless communication system.
According to one known roaming method, once the wireless device passes through an interference region or a coverage hole such that acquisition with the more-preferred stored SID element is temporarily unavailable, the wireless device seeks to acquire another stored SID element in the PRL. The wireless device first attempts acquisition according to a first SID acquisition sequence. The first SID acquisition sequence includes repeatedly attempting acquisition of the more-preferred stored SID element. If the wireless device is unable to acquire the more-preferred stored SID element according to the first SID acquisition sequence, and the wireless device acquires a less-preferred stored SID element in the PRL, then the wireless device attempts acquisition according to a second SID acquisition sequence. The second SID acquisition sequence includes single sequential acquisition attempts of each stored SID element in the PRL. As a result, according to the second acquisition sequence, the wireless device makes a single attempt to acquire the more-preferred stored SID element listed in the PRL during the second acquisition sequence.
If the wireless device fails to acquire the at least one more-preferred SID element, and acquires the at least one less-preferred stored SID element in the PRL, then the wireless device registers as a roamer and camps on the less-preferred stored SID element. As a result, if the user initiates a call or receives a call, then the call will be serviced on the less-preferred stored SID element, even if the wireless device emerges from the interference region or coverage hole and acquisition may be available on one of the more-preferred stored SID elements. For example, by the time the wireless device initiates or receives a call after acquisition of the less-preferred stored SID element, the wireless device may have traveled outside of the coverage hole or the interference region. As a result, the wireless device camps on the less-preferred stored SID element and not on the more-preferred stored SID element. The user may possibly incur roaming charges, although the wireless device is actually within the coverage area corresponding to the more-preferred stored SID element.